1. Field of the Invention
The invention relates in general to Fizeau interferometers for optical testing of spherical surfaces.
2. Description of the Prior Art
In order to test optical components with non-zero power using a Fizeau interferometer, a precision Fizeau lens called a “transmission sphere” must be used. A key advantage of the Fizeau interferometer is that, in its normal configuration, both the test and reference beams pass along a common optical path within the interferometer. As a result, only a single reference surface needs to be made very accurately. The optics in the transmission sphere are designed to operate at a specific wavelength so that the transmitted wavefront contains rays nominally perpendicular to an accurate reference surface (i.e., the reference surface and the spherical wavefront produced by the transmission sphere have the same curvature).
Most commercial transmission spheres are designed for operation at the He—Ne laser wavelength of 632.8 nm. As the use of high-power solid-state lasers becomes more common in laser interferometers, there is often a need to use transmission spheres operating at different wavelengths, ranging, for example, from as low as 500-nm to the higher 637 nm to 670 nm approximate range, depending on the laser used as the light source. If the transmission sphere is used at a wavelength that is different from its design wavelength, aberrations are introduced into the transmitted wavefront. The most significant aberration that results from such a mismatch in wavelengths is a change in the optical power (also called chromatic focus shift).
Chromatic focus shift change causes two problems. First, the return beams are brought into focus at a different point within the instrument; that is, they are displaced from their nominal focus position (referred to as “defocusing” of the beams). The test beam can be refocused by shifting the test surface along the optical axis of the instrument, but that is not possible for the reference beam because of the fixed position of the reference surface in relation to the other optical elements in the transmission sphere. Thus, when the interferometer uses a field stop, the defocusing of the return beams can cause the reference beam to be partially blocked by the stop, which renders the system unusable. The second problem is that the light rays will not be nominally perpendicular to the reference surface and, as a result, the test and reference beams will no longer travel along a common path through the lens. This causes significant errors in the test results.
Therefore, it would be very desirable to have the ability to introduce a power correction into the optics transmitting the illumination beam in order to adjust for chromatic focus shifts in the transmission sphere caused by the use of different wavelengths at the source. This would allow, for instance, the use of a transmission lens designed for a specific wavelength, such 633 nm, at a different wavelength, such as 658 nm. The present invention provides a simple and inexpensive way to achieve this objective.